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We present the results of an experimental study of a tetragonal-to-orthorhombic phase transition with rather high transition temperature (T_TO ~380 K) in the clathrate material BaNi₂P₄. The transition is accompanied by formation of structural domain patterns which are revealed in polarized optics studies. The electrical resistivity shows metallic behavior both above and below T_TO. The transition temperature is notably affected by the low-temperature (20 K) irradiation with relativistic 2.5 MeV electrons. Additional insight into the nature of the transition is obtained from Hall effect measurements and band structure calculations. Similarity to the behavior found in the parent compounds of iron based superconductors will be discussed.